sahar mosleh california state university san marcospage 1 a for loop can contain multiple...

Sahar Mosleh California State University San Marcos

• A for loop can contain multiple initialization actions separated with commas

• Caution must be used when combining a declaration with multiple actions

• It is illegal to combine multiple type declarations with multiple actions, for example

• To avoid possible problems, it is best to declare all variables outside the for statement

• A for loop can contain multiple update actions, separated with commas, also

• It is even possible to eliminate the loop body in this way

• However, a for loop can contain only one Boolean expression to test for ending the loop

The Comma in for Statements


Infinite Loops

• A while, do-while, or for loop should be designed so that the value tested in the Boolean expression is changed in a way that eventually makes it false, and terminates the loop

• If the Boolean expression remains true, then the loop will run forever, resulting in an infinite loop

• Loops that check for equality or inequality (== or !=) are especially prone to this error and should be avoided if possible


Loop Bugs• The two most common kinds of loop errors are unintended infinite loops and

off-by-one errors

• An off-by-one error is when a loop repeats the loop body one too many or one too few times

• This usually results from a carelessly designed Boolean test expression

• Use of == in the controlling Boolean expression can lead to an infinite loop or an off-by-one error

• This sort of testing works only for characters and integers, and should never be used for floating-point


Example:

Number = 2;While (number !=12){system.out.println(number); //Ok since printing even numbers and number riches 12Number = number +2;}

----------------------------------------------------------------Number = 1;While (number !=12){ system.out.println(number); //Not Ok since printing odd numbers and number never riches 12Number = number +2;}

----------------------------------------------------------------------------Number = 2;While (number <12){ system.out.println(number); //Ok for odd and even numbersNumber = number +2;}


Local Variables

• A variable declared within a method definition is called a local variable

• All variables declared in the main method are local variables

• All method parameters are local variables

• If two methods each have a local variable of the same name, they are still two entirely different variables


Blocks

• A block is another name for a compound statement, that is, a set of Java statements enclosed in braces,{}.

• A variable declared within a block is local to that block, and cannot be used outside the block

• Once a variable has been declared within a block, its name cannot be used for anything else within the same method definition


Declaring Variables in a for Statement• You can declare one or more variables within the

initialization portion of a for statement

• A variable so declared will be local to the for loop, and cannot be used outside of the loop

• If you need to use such a variable outside of a loop, then declare it outside the loop


Assignment Compatibility

• In general, the value of one type cannot be stored in a variable of another type

• int intVariable = 2.99; //Illegal

• The above example results in a type mismatch because a double value cannot be stored in an int variable

• However, there are exceptions to this

• double doubleVariable = 2;

• For example, an int value can be stored in a double type


Assignment Compatibility• More generally, a value of any type in the following list can be assigned to

a variable of any type that appears to the right of it

• byteshortintlongfloatdouble• Char

• Note that as your move down the list from left to right, the range of allowed values for the types becomes larger

• An explicit type cast is required to assign a value of one type to a variable whose type appears to the left of it on the above list (e.g., double to int)

• Note that in Java an int cannot be assigned to a variable of type boolean, nor can a boolean be assigned to a variable of type int


Arithmetic Operators and Expressions

• As in most languages, expressions can be formed in Java using variables, constants, and arithmetic operators

• These operators are + (addition), - (subtraction), * (multiplication), / (division), and % (modulo, remainder)

• An expression can be used anyplace it is legal to use a value of the type produced by the expression


Arithmetic Operators and Expressions

• If an arithmetic operator is combined with int operands, then the resulting type is int

• If an arithmetic operator is combined with one or two double operands, then the resulting type is double

• If different types are combined in an expression, then the resulting type is the right-most type on the following list that is found within the expression

• byteshortintlongfloatdouble• Char


Integer and Floating-Point Division• When one or both operands are a floating-point type, division results in a

floating-point type

• 15.0/2 evaluates to 7.5

• When both operands are integer types, division results in an integer type

• Any fractional part is discarded • The number is not rounded

• 15/2 evaluates to 7

• Be careful to make at least one of the operands a floating-point type if the fractional portion is needed


The % Operator

• The % operator is used with operands of type int to recover the information lost after performing integer division

• 15/2 evaluates to the quotient 7• 15%2 evaluates to the remainder 1

• The % operator can be used to count by 2's, 3's, or any other number

• To count by twos, perform the operation number % 2, and when the result is 0, number is even


Type Casting• A type cast takes a value of one type and produces a value of another type

with an "equivalent" value

• If n and m are integers to be divided, and the fractional portion of the result must be preserved, at least one of the two must be type cast to a floating-point type before the division operation is performed

• double ans = n / (double)m;

• Note that the desired type is placed inside parentheses immediately in front of the variable to be cast

• Note also that the type and value of the variable to be cast does not change


More Details About Type Casting

• When type casting from a floating-point to an integer type, the number is truncated, not rounded

• (int)2.9 evaluates to 2, not 3

• When the value of an integer type is assigned to a variable of a floating-point type, Java performs an automatic type cast called a type coercion

• double d = 5;

• In contrast, it is illegal to place a double value into an int variable without an explicit type cast

• int i = 5.5; // Illegal• int i = (int)5.5 // Correct

sahar mosleh california state university san marcospage 1 a for loop can contain multiple...

Documents